This invention relates generally to test, measurement instruments, and systems for characterizing multipair telecommunications circuits where twisted pairs of insulated conductors are stranded together to form the core of a communications cable.
Due to the proximity of the conductors to each other, the composition of materials used to insulate the conductors, and the composition of materials used to insulate and shield the cable core, various forms of capacitance appear normally between the conductors of a twisted pair and other elements of the cable. These capacitances typically occur in the four forms of: (1) capacitance--mutual (CM) between the two wires of the twisted pair; (2) capacitance--unbalance-pair-to-ground (CUPG); (3) capacitance--unbalance-pair-to-shield (CUPS); and (4) capacitance--unbalance-pair-to-pair (CUPP).
These capacitances must be controlled to achieve high fidelity communication with minimum noise. Signal degredation is associated with each capacitance, as follows. The mutual capacitance (CM) attentuates the voice signal, and therefore affects the ability to hear on a telephone line. The capacitance unbalance of a twisted pair to ground (CUPG), and the capactance unbalance to shield (CUPS), must be controlled in order to prevent crackling noises from appearing on the telephone circuits due to power line harmonic induction. The capacitance unbalance pair to pair (CUPP) must be controlled in order to prevent crosstalk between telephone circuits in the voice frequency band of 200 to 3000 Hz.
Existing measurement systems have been designed for use in the cable factory, not for installed telephone cables and plants in the field. These systems are too large and cumbersome and are, therefore, very different if not impossible to transport. Conventional systems for measuring telephone cable CM, CUPG, CUPS, and CUPP include a sinusoidal excitation approach known as the Bridge Technique, and the Constant Current Charging RC Technique.
The Bridge Technique can only measure short lengths of cable, and therefore is not suitable for long lengths in the outside plant. In addition, when the instrument is coupled to the cable under test, knobs must be adjusted to balance the cable capacitance against a standard capacitator; this requires comparison of unlike quantities because the capacitor is a lumped capacitance, while the cable is a distributed capacitance. Accordingly, this technique is cumbersome, time consuming, and offers ample opportunity for human error. Many switching functions are required, thus making for an instrument which wears out and is less rugged than desired. In addition, this technique is subject to phase shift and signal delay which is inherent in transmission lines and cables; this results in erroneous readings, especially in "lossey" dielectrics.
The Constant Current RC Charging Technique has the disadvantage of pulsing a large initial transient signal for inductive loads. Also, it has the disadvantage of requiring that the load be coupled to the system being measured as part of the feedback path. This approach lacks predictable frequency spectrum response, and is subject to power line and radio frequency (RF) noise. Additionally, the capacitance measurement is affected by the individual presence of series resistance and/or parallel resistance which produces a shunt conductance; the simultaneous presence of both types of resistances has a second-order effect also in lossey dielectrics; erroneous readings will be obtained.
In practically all conventional cases where capacitance measurement circuits are described for measuring these capacitances, it is recommended that distributed capacitance circuits be added; however, this has been difficult to achieve. For example, in the pair to ground case, previous techniques must reference ground and therefore provide unstable readings.